Yufei W. Blankenship

Link error prediction methods for multicarrier systems

Multicarrier modulations such as OFDM with adaptive modulation and coding (AMC) are well suited for high data rate broadband systems that operate in multipath environments and are considered as promising candidates for future generation cellular systems (e.g., 4G). Cellular system performance is normally investigated with system level simulations that are computationally complex. For broadband multicarrier systems, incorporating a detailed physical layer emulator into the system simulator becomes impractical, so there is a need for simplified link performance predictors. However, due to the large variability of the channel in the frequency domain, two links with the same average SNR can experience drastically different performance, thus making it difficult to accurately predict the instantaneous link performance such as the frame error rate. In this paper, the accuracy of two FER prediction methods is studied: Packet error rate indicator (PER-indicator) and exponential effective SIR mapping (Exp-ESM). Both methods are shown to have accuracy within a few tenths of a dB under a wide range of modulation schemes, coding rates and channel types. These methods are then extended to handle more advanced link enhancements such as hybrid ARQ and Alamouti encoding. The Exp-ESM method has slightly better accuracy than the PER-indicator, and is the preferred link error predictor for a system simulator.

Downlink Control Channel Design for 3GPP LTE

With the emergence of packet-based wireless broadband systems, work has begun on long term evolution (LTE) of the UMTS Terrestrial Radio Access and Radio Access Network aimed for commercial deployment in 2010. Goals for the evolved system include support for improved system capacity and coverage, high peak data rates, low latency, reduced operating costs, multi-antenna support, flexible bandwidth operations and seamless integration with existing systems. To reach these goals, a new design for the air interface including control channel is envisioned. This paper provides a preliminary look at an efficient downlink control channel design to reduce the overhead required to support data transmission. Initial performance results show that close to optimal system performance can be achieved with downlink control overhead of less than 14%.

ARP and QPP Interleavers for LTE Turbo Coding

The Long Term Evolution (LTE) of the WCDMA standard requires turbo decoding throughputs of up to 100 Mbps. Unfortunately, the existing WCDMA turbo interleaver does not permit an efficient high throughput implementation. The almost regular permutation (ARP) and quadratic polynomial permutation (QPP) interleavers were proposed to rectify this situation with QPP selected for LTE. In this paper, the interleavers are compared and a full suite of LTE designs and performance results for both are presented.

A Primer on 3GPP Narrowband Internet of Things

Narrowband Internet of Things (NB-IoT) is a new cellular technology introduced in 3GPP Release 13 for providing wide-area coverage for IoT. This article provides an overview of the air interface of NB-IoT. We describe how NB-IoT addresses key IoT requirements such as deployment flexibility, low device complexity, long battery lifetime, support of massive numbers of devices in a cell, and significant coverage extension beyond existing cellular technologies. We also share the various design rationales during the standardization of NB-IoT in Release 13 and point out several open areas for future evolution of NB-IoT.

Error prediction for adaptive modulation and coding in multiple-antenna OFDM systems

In this paper, the problem of packet error rate (PER) prediction is addressed in the multiple-antenna broadband OFDM context, and its impact on adaptive modulation and coding (AMC) is quantified. The analysis is based on a physical layer comprising various modulation and coding schemes, ranging from robust space-time block coding (STBC) modes to high bit rate spatial division multiplexing (SDM) modes, and also hybrid SDM-STBC schemes. For each mode the expression of several link quality metrics (LQM) enabling PER prediction in the broadband OFDM channel, such as instantaneous signal-to-noise ratio (SNR), capacity, or exponential effective SNR metrics are provided. Their advantages and limitations are investigated. Finally, their performance is benchmarked in the IEEE 802.11a/g/n context. It is shown that the choice of the LQM has a significant impact on the throughput performance of the AMC algorithm.

Efficient OFDM-HARQ system evaluation using a recursive EESM link error prediction

OFDM system-level simulations require a performance estimate for each link, but it is typically infeasible to conduct simultaneous link-level simulations for multiple users within a reasonable amount of time. Therefore, it is critical to have a simple link error probability predictor that accurately models coded OFDM performance. The exponential ESM (EESM) method was shown to be a very good link error prediction method for multi-carrier systems in the case of single transmission. Here, the EESM method is extended to cover hybrid ARQ (HARQ) techniques: Chase combining, incremental redundancy (IR) or an arbitrary combination of both. Further simplifications are introduced to greatly reduce the amount of memory necessary to utilize EESM for modeling OFDM-HARQ transmissions

Enhanced Link Adaptation Methods for Wireless Multi-Carrier Systems

This work describes a family of low feedback, high performance link adaptation algorithms for multi-carrier systems. The methods include the use of the exponential effective SNR mapping (EESM) method as part of the modulation/coding selection process. The drawbacks of current link adaptation algorithms are reviewed, and means of combating the effect of Doppler and fading gain variation are proposed. In addition, a method enabling conventional SNR feedback reports to be converted to the effective SNR domain is described. This allows the effect of variable power allocation on coded BER to be more accurately estimated. Simulation results show that the proposed methods are effective.

Aggressive modulation/coding scheme selection for maximizing system throughput in a multi-carrier system

Multi-carrier modulation techniques, such as OFDM, are attractive propositions for the design of future broadband wireless communication systems due to higher spectral efficiency than CDMA. A capacity enhancing technique to be commonly used in these systems is adaptive modulation/coding (AMC) for link adaptation. With AMC, the modulation and coding scheme (MCS) is selected based on the channel quality information. MCS selection is an imperfect process by nature, due to, for example, the variability of the channel. Moreover, since the set of available MCSs is quantized, it is not always possible to select the optimal rate. Using these non-optimal MCS values results in reduced system capacity. This paper describes a method that attempts to maximize system and user throughputs in a multi-carrier system by overcoming the impacts of such non-optimal MCS selection through the use of hybrid ARQ (HARQ). This method relies on aggressive MCS selection and multiple HARQ transmissions. The performance impact of this method on various types of packet data applications is evaluated by system level simulation. Simulation results indicate that significant system and user throughput gains can be achieved by this method.

Block product code design with the aid of union bounds

Applying soft-input soft-output iterative decoding allows block product codes (BPCs) to exhibit error-correcting capability close to that of convolutional code-based turbo codes. However, finding the best BPC design for a given target code size can be difficult. This paper analyzes BPC performance using weight distribution and union bounds, which leads to a systematic BPC design procedure. The analysis is supported by simulations and examples.

Unequal error protection design using convolutional codes

The introduction of adaptive multi-rate (AMR) speech coders in wireless standards, such as GSM, imposes new challenges for forward error correction (FEC) design. Traditional FEC techniques of providing unequal error protection (UEP) may not maximize performance for a given set of design requirements, such as complexity limits. This paper introduces a new FEC technique that improves error correction performance while satisfying complexity constraints. This technique is applied to the new wideband AMR standard for a performance and complexity comparison.